Holes are drilled in the earth for many reasons, for example, in the oil and gas industry, holes are drilled to access sub-surface hydrocarbon reservoirs. In recent years, in the oil and gas industry the main focus has been on the drilling of “directional” holes or wells. Directional drilling is particularly useful in offshore operations, allowing a large area to be accessed from a single fixed location platform. However, there remains a requirement to drill vertical wells, and there are a number of reasons why accurate vertical drilling may be required. For example, to maximise the number of wells in a seabed template, below which a number of holes may extend, the initial vertical hole sections must be drilled with minimal deviation to maintain anti-collision safety margins. Accurate vertical drilling also facilitates the drilling of “lean” profile wells, where the hole-lining casing diameter is much closer to the open hole diameter, and close tolerance casing strings may be used. Also, even when drilling a deviated hole, accurate vertical drilling is important to minimise deviation and dogleg severity in the initial sections of the well, in order to ensure that when drilling the deeper deviated hole sections the torque and drag does not increase to unacceptable levels. The absence of unnecessary or unintended deviation and doglegs is also important in reducing casing wear due to sucker rod action during subsequent production operations.
At present, accurate vertical drilling relies on use of expensive tools which rely on sophisticated rotary steerable technology.
It is among the objectives of particular embodiments of the present invention to provide a relatively simple drilling apparatus which may be used to provide accurate vertical drilling.